1. Field of the Invention
The present invention relates to an inkjet head, and more specifically, to an inkjet head that discharges droplets from micro nozzles in accordance with a pressure change generated by applying electric power to a piezoelectric element provided at an individual ink chamber to form a pattern.
2. Description of the Related Art
For an inkjet head that forms a pattern by discharging micro droplets, plural types of products are provided where a pressure change is generated in an individual ink chamber. For example, there are a thermal inkjet type in which a heater is set in the individual ink chamber to vaporize liquid to cause a pressure change and a type using an actuator set in the individual ink chamber. For the type using an actuator, based on the type of actuators, a piezoelectric element type or an electrostatic type may be included.
For the type using an actuator, although it is capable to use ink of a wide variety of physical properties, there has been a problem in making high density individual ink chambers for downsizing a head. However, by using a so-called Micro Electro Mechanical Systems (MEMS) process, a technique to make high density individual ink chambers has been established. In this process, a unimorph type actuator is provided at each of the individual ink chambers by forming a stacked structure of a vibration layer, electrodes, a piezoelectric layer or the like using a semiconductor device manufacturing process such as a thin layer forming technique and patterning each of the piezoelectric elements or electrodes using a semiconductor device manufacturing process such as a photolithography to make high density individual ink chambers.
When forming electrodes or interconnects by the thin layer forming technique, as a layer of a metal or an alloy is formed by sputtering, Chemical Vapor Deposition (CVD), or the like it is difficult to form a thick layer. Concretely, it is difficult to form a layer whose thickness is more than or equal to 5 μm. Generally, it is necessary to have the thickness of the layer less than or equal to 1 μm in the light of a layer stress or manufacturing efficiency (process time) when the thin layer forming technique is used. Therefore, when the thin layer forming technique is used, in order to reduce an electric resistance of the electrodes or the interconnects, it is necessary to broaden the dimension of the electrode or the interconnects. As a result, the size of the head becomes larger and the number of chips obtained from a single wafer becomes smaller to costs to go up.
For an inkjet head, printing speed can be increased by increasing the density, in other words, increasing the number of nozzles provided for the inkjet head, so that the number of discharging dots for each scan can be increased. Therefore, it is necessary to increase the number of nozzles, in other words, the number of piezoelectric elements, provided per head. The nozzles or the piezoelectric elements are aligned in a predetermined direction in the head. When a large number of nozzles are provided per head, generally, a common electrode interconnect by which voltage is commonly applied to the entirety of the piezoelectric elements is formed to extend in the predetermined direction in which the piezoelectric elements or the nozzles are aligned and the voltage is applied from both sides of the common electrode interconnect. However, when the resistance of the common electrode interconnect is high, voltage drop occurs especially near the center. For nozzles near the center, since not enough voltage is applied to the corresponding piezoelectric elements, droplets cannot be properly discharged which deteriorates uniform discharging. When the common electrode interconnect is formed by the thin layer forming technique, the thickness of the common electrode interconnect becomes thinner as described above and the resistance value becomes higher so that the above voltage drop occurs remarkably. Therefore, there is a problem that the high density of nozzles obtained by the semiconductor device manufacturing process cannot be utilized because of the high resistance of the common electrode interconnect.
It is disclosed in Patent Document 1, a technique related to an inkjet head in which a common electrode interconnect is formed to extend in a direction in which individual ink chambers are aligned at one side of the individual ink chambers where individual electrode interconnects corresponding to the individual ink chambers are provided at other side of the individual ink chambers, the common electrode interconnect is further extended to the sides of the individual electrodes, and the common electrode interconnect is formed by metal having a low value of resistance to improve discharging uniformity. Further, according to Patent Document 1, concave portions are formed between the common electrode interconnect and the individual ink chambers to release the stress generated when piezoelectric elements are actuated.
It is disclosed in Patent Document 2, a technique related to an inkjet head in which an ink channel is formed to have the same height as that of an individual ink chamber and a piezoelectric layer is extended above the ink channel to strengthen the structure and prevent damage such as cracking or the like to a vibration layer formed on the ink channel.
It is disclosed in Patent Document 3, a technique related to an inkjet head in which a connecting interconnect layer electrically connected to a lower electrode (which is a common electrode) is formed to extend in a direction in which ink chambers are aligned to lower the resistance of the common electrode and to reduce variation in discharging.
It is disclosed in Patent Document 4, a technique related to an inkjet head in which a stacked electrode electrically connected to a lower electrode (which is a part of a common electrode) is formed at an outside area of individual ink chambers as the common electrode to lower the resistance of the common electrode and to reduce variation in discharging. Further, according to Patent Document 4, a stress releasing layer having a small thermal expansion coefficient (larger than that of a vibration layer and smaller than that of the stacked electrode) is provided at an end of the stacked electrode to prevent the removal of the stacked electrode by high temperature during manufacturing so that the damage to the vibration layer can also be prevented.